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M.H. Perrott
Analysis and Design of Analog Integrated CircuitsLecture 12
Feedback
Michael H. PerrottMarch 11, 2012
Copyright © 2012 by Michael H. PerrottAll rights reserved.
M.H. Perrott
Open Loop Versus Closed Loop Amplifier Topologies
Open loop – want all bandwidth limiting poles to be as high in frequency as possible
Closed loop – want one pole to be dominant and all other parasitic poles to be as high in frequency as possible
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VoutZsrc
Vsrc
SourceAmp VoutZsrc
Vsrc
SourceAmp
Zf
Open Loop Closed Loop
Vout/Vin
(dB)
w (rad/s)wbw w1 w2
Vout/Vin
(dB)
w (rad/s)wbw w1 w2
VinVin
M.H. Perrott
Consider an Open Loop Integrator
Parameterize integrator in terms of its unity gain frequency, wunity rad/s- Define H(s) = wunity/s- Note that |H(wunity)| = 1
3
Vout
Vout/Vin(dB)
w (rad/s)wunity
sVin
wunity
0 dB
H(w)
H(s)Vin Vout
M.H. Perrott
Now Surround the Integrator with a Feedback Path
The feedforward path is H(s) = wunity/s The feedback path is formed by Z1 and Z2
Derivation of closed loop transfer function:
4
VoutZ1
Z2
Vx
Vin
Vout/(Vin-Vx)(dB)
w (rad/s)wunity
0 dB
H(w)
20log
H(s)
Vout − VxZ2
=Vx − 0Z1
and Vout = H(s)(Vin − Vx)
⇒ VoutVin
=H(s)
1 + Z1/(Z1 + Z2)H(s)
M.H. Perrott
Observations of Impact of Feedback
Define = Z1/(Z1+Z2) and rewrite transfer function as:
At low frequencies: At high frequencies:
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Vout/Vin
w (rad/s)wunity
0 dB
H(w)
20log(1/β)
wbw
20log
VoutZ1
Z2
Vx
Vin
H(s)
VoutVin
=H(s)
1 + Z1/(Z1 + Z2)H(s)=
H(s)
1 + β ·H(s)
¯̄̄̄VoutVin
¯̄̄̄s→0
=∞
1 + β ·∞ =1
β
¯̄̄̄VoutVin
¯̄̄̄s→∞
= |H(w)|
(since |β ·H(w)| ¿ 1)
M.H. Perrott
Vout/Vin
w (rad/s)wunity
0 dB
H(w)
20log(1/β)
wbw
20log
VoutZ1
Z2
Vx
Vin
H(s)
H(s)
β
Vin Vout
General View of Feedback
Closed loop transfer function:
- This is called Black’s formula
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VoutVin
=H(s)
1 + β ·H(s)
¯̄̄̄VoutVin
¯̄̄̄s→∞
= |H(w)|
At low frequencies: At high frequencies:¯̄̄̄VoutVin
¯̄̄̄s→0
=1
β
M.H. Perrott
The feedback path sets the closed loop gain at low frequencies- Assumes the open loop gain is large at low frequencies- Implies that accurate closed loop gain can be achieved
at low frequencies despite variations in open loop gain The feedback path also influences the closed loop
bandwidth
Vout/Vin
w (rad/s)wunity
0 dB
H(w)
20log(1/β)
wbw
20log
H(s)
β
Vin Vout
General Observations of Feedback
7
VoutVin
=H(s)
1 + β ·H(s)
M.H. Perrott
The low frequency gain is:
The bandwidth roughly corresponds to:
Vout/Vin
w (rad/s)wunity
0 dB
H(w)
20log(1/β)
wbw
20log
H(s)
β
Vin Vout
Gain Bandwidth Product for Closed Loop Systems
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VoutVin
=H(s)
1 + β ·H(s)
|H(wbw )| ≈1
β
¯̄̄̄VoutVin
¯̄̄̄s→0
=1
β
⇒¯̄̄̄wunitywbw
¯̄̄̄≈ 1
β⇒ wunity =
1
β· wbwFor H(s) =
wunitys
Closed loop systems exhibit constant gain-bandwidth productset by the unity gain frequency of the open loop amplifier
M.H. Perrott
The low frequency gain is:
The bandwidth roughly corresponds to:
Gain bandwidth product is wunity:
Vout/Vin
w (rad/s)wunity
0 dB
H(w)
20log
H(s)
β = 1
Vin Vout
Example: Unity Gain Amplifier
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VoutVin
=H(s)
1 +H(s)
|H(wbw )| ≈ wunity
¯̄̄̄VoutVin
¯̄̄̄s→0
= 1
Unity gain closed loop amplifiers maximize theclosed loop bandwidth assuming closed loop gain ≥ 1
1 · wunity = wunity
M.H. Perrott
Let us now model H(s) as:
To first order, the closed loop bandwidth and gain are relatively unchanged
Vout/Vin
w (rad/s)wunity
0 dB
H(w)
20log
H(s)
β = 1
Vin Vout
wdominant
20log(K)
Issue: Open Loop Amplifiers have Finite DC Gain
10
VoutVin
=H(s)
1 +H(s)
H(s) =K
1 + s/wdominant
What is the impact of having finite, open loop, DC gain?
M.H. Perrott
H(s) =K
1 + s/wdominant
For unity gain configuration of closed loop amplifier:
We see that finite open loop DC gain leads to a slight reduction of the closed loop DC gain- We want K >> 1 for the unity gain closed loop amplifier
Vout/Vin
w (rad/s)wunity
0 dB
H(w)
20log
H(s)
β = 1
Vin Vout
wdominant
20log(K)
Further Examination of Finite, Open Loop, DC Gain
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VoutVin
=H(s)
1 +H(s)
¯̄̄̄VoutVin
¯̄̄̄s→0
=H(s)
1 +H(s)
¯̄̄̄s→0
=K
1 +K=
1
1 + 1/K
M.H. Perrott
H(s) =K
1 + s/wdominant
For general configuration of closed loop amplifier:
Finite open loop DC gain still leads to reduction of closed loop DC gain- We want K >> 1/ in this case- We will see implications of this issue later in the class
Vout/Vin
w (rad/s)wunity
0 dB
H(w)
20log(1/β)
wbw
20log
H(s)
β
Vin Vout20log(K)
More General View of Finite, Open Loop, DC Gain
12
VoutVin
=H(s)
1 + β ·H(s)
¯̄̄̄VoutVin
¯̄̄̄s→0
=K
1 + β ·K =
µ1
β
¶1
1 + (1/β)/K
M.H. Perrott
Practical amplifiers have non-dominant poles, too:
- Of course, there can be multiple parasitic poles and also zeros
A key issue of such parasitic poles is their influence on the stability of the closed loop amplifier
Vout/Vin
w (rad/s)
H(w)
20log(1/β)
wbw
20log
H(s)
β
Vin Vout20log(K)
wp
H(s) =
µK
1 + s/wdominant
¶µ1
1 + s/wp
¶
The Issue of Parasitic Open Loop Poles
13
VoutVin
=H(s)
1 + β ·H(s)
M.H. Perrott
We define the open loop response, A(s), as:
Note that the unity gain frequency, w0 , of A(w) is approximately the same as the closed loop bandwidth, wbw
- Looking at the plot above, we can see that the intersection of |H(w)| and 1/ corresponds to the closed loop bandwidth, wbw
|A(w0)| = 1 ⇒ β · |H(w0)| = 1 ⇒ |H(w0)| = 1/β
Vout/Vin
w (rad/s)
H(w)
20log(1/β)
wbw
20log
H(s)
β
Vin Vout20log(K)
wp
A(s) = β ·H(s)
Key Tool for Assessing Stability: Open Loop Response
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VoutVin
=H(s)
1 + β ·H(s)
M.H. Perrott
Stability Analysis Based on Phase Margin of A(w)
Phase margin is a key metric when examining the stability of a system- Phase margin is defined as 180° + phase{A(w0)}
w0 corresponds to the unity gain frequency of the open loop response (i.e., |A(w0)| = 1)
w0 is approximately the same as the closed loop bandwidth, wbw- Phase margin must be greater than 0 degrees for the
closed loop system to be stable Typically want phase margin to be greater than 45°
Key skill: you must be able to plot Bode plots in both magnitude and phase!
15
M.H. Perrott
Review of Bode Plot Basics
Example:
- Log of magnitude (dB):
Taking the log allows the poles and zeros to be plotted separately and then added together
- Phase:
Phase of poles and zeros can also be plotted separately and then added together
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A(w) =1 + jw/wz
(1 + jw/wp1)(1 + jw/wp2)
20 log |A(w)|= 20 log |1 + jw/wz |− 20 log |1 + jw/wp1|− 20 log |1 + jw/wp2|
6 A(w)
= 6 (1 + jw/wz)− 6 (1 + jw/wp1)− 6 (1 + jw/wp2)
M.H. Perrott
Review: Plotting the Magnitude of Poles
Plot the magnitude response of pole wp1
- For w << wp1:
- For w >> wp1:
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20 log |Ap1(w)| = 20 log¯̄̄̄
1
1 + jw/wp1
¯̄̄̄= −20 log |1 + jw/wp1|
20 log |Ap1(w)| ≈ −20 log |1| = 0
20 log |Ap1(w)| ≈ −20 log |w/wp1|
ω
20log|Ap1(ω)|
0 dB
-20 dB/decadeωp1
M.H. Perrott
Plotting the Phase of Poles
Plot the phase response of pole wp1
- For w << wp1:
- For w = wp1:
- For w >> wp1:
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6 Ap1(w) ≈ − arctan (0) = 0◦
ω
Ap1(ω)
0o ωp1ωp1/10 ωp1∗10
-45o
-90o
6 Ap1(w) = − 6 (1 + jw/wp1) = − arctan (w/wp1)
6 Ap1(w) ≈ − arctan (1) = −45◦
6 Ap1(w) ≈ − arctan (∞) = −90◦
M.H. Perrott
Review: Plotting the Magnitude of Zeros
Plot the magnitude response of zero wz
- For w << wz:
- For w >> wz:
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20 log |Az(w)| = 20 log |1 + jw/wz|
20 log |Az(w)| ≈ 20 log |1| = 0
20 log |Az(w)| ≈ 20 log |w/wz |
ω
20log|Az(ω)|
0 dB
20 dB/decade
ωz
M.H. Perrott
Plotting the Phase of Zeros
Plot the phase response of zero wz
- For w << wz:
- For w = wz:
- For w >> wz:
20
6 Az(w) ≈ arctan (0) = 0◦
ω0oωzωz/10 ωz∗10
45o
90oAz(ω)
6 Az(w) = 6 (1 + jw/wz) = arctan (w/wz)
6 Az(w) ≈ arctan (1) = 45◦
6 Az(w) ≈ arctan (∞) = 90◦
M.H. Perrott
Example of Closed Loop Stability Evaluation
Consider the case where:
- This implies that:
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Vout/Vin
w (rad/s)
H(w)
20log(1/β)
wbw
20log
H(s)
β
Vin Vout
wp
VoutVin
=H(s)
1 + β ·H(s)
H(s) =
µK
s
¶µ1
1 + s/wp
¶
A(s) = β
µK
s
¶µ1
1 + s/wp
¶
M.H. Perrott
Phase Margin Versus Open Loop Gain
Note the closed loop pole locations versus open loop gain- Is the closed loop system unstable for any case above?
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-90o
-180o
-120o
-150o
20log|A(f)|
f
angle(A(f))
Open loopgain
increased
0 dB
PM = 33o for C
PM = 45o for B
PM = 59o for A
A
A
A
B
B
B
C
C
C
Evaluation ofPhase Margin
Closed Loop PoleLocations
Dominantpole pair
fp
Re{s}
Im{s}
0
M.H. Perrott
Corresponding Closed Loop Behavior
Frequency response sees more peaking with higher open loop gain- How does this relate to the movement of the closed loop pole
locations? Step response see more ringing with higher open loop gain
- How does this relate to the closed loop frequency response? 23
5 dB0 dB
-5 dB
f
A
C
fp
B
Closed Loop Frequency Response
1.4
0
1
0.6
t
AB
C
Closed Loop Step Response
M.H. Perrott
Some Key Observations
We have seen that increasing the open loop gain of A(w)leads to higher closed loop bandwidth- How is this consistent with the statement that increasing
closed loop gain leads to lower closed loop bandwidth? As an exercise, consider the impact of the following:
- Keep unchanged and increase the open loop gain of H(w)- Keep H(w) unchanged and increase
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Vout/Vin
w (rad/s)
H(w)
20log(1/β)
wbw
20log
H(s)
β
Vin Vout
wp
VoutVin
=H(s)
1 + β ·H(s)A(s) = β ·H(s)
M.H. Perrott
Example 2 of Closed Loop Stability Evaluation
Consider the case where:
- This implies that:
25
Vout/Vin
w (rad/s)
H(w)
20log(1/β)
wbw
20log
H(s)
β
Vin Vout
wp1,wp2,wp3
VoutVin
=H(s)
1 + β ·H(s)
H(s) =
µK
s
¶µ1
1 + s/wp1
1
1 + s/wp2
1
1 + s/wp3
¶
A(s) = β
µK
s
¶µ1
1 + s/wp1
1
1 + s/wp2
1
1 + s/wp3
¶
M.H. Perrott
Phase Margin Versus Open Loop Gain
Note the closed loop pole locations versus open loop gain- Is the closed loop system unstable for any case above?
26
-90o
-315o
-165o
-180o
-240o
20log|A(f)|
ffp3
angle(A(f))
Open loopgain
increased
0 dB
PM = 51o for B
PM = -12o for C
PM = 72o for A
Non-dominantpoles
Dominantpole pair
ABC
B
A
A
B
C
C
Evaluation ofPhase Margin
Closed Loop PoleLocations
fp1 fp2
Re{s}
Im{s}
0
M.H. Perrott
Corresponding Closed Loop Behavior
Frequency response again sees more peaking with higher open loop gain- How does this relate to the movement of the closed loop pole
locations? Step response ringing grows for high open loop gain
- How does this relate to the closed loop pole locations? 27
0 dB
Closed Loop Frequency Response Closed Loop Step Response
1
TimeFrequency
A
C
B
CB
A
M.H. Perrott
Open Loop Versus Closed Loop Amplifier Topologies
Now that we understand the phase margin criterion, can you explain why amplifiers designed to be within a closed loop system should have one dominant pole that is much lower in frequency than the parasitic poles?
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VoutZsrc
Vsrc
SourceAmp VoutZsrc
Vsrc
SourceAmp
Zf
Open Loop Closed Loop
Vout/Vin
(dB)
w (rad/s)wbw w1 w2
Vout/Vin
(dB)
w (rad/s)wbw w1 w2
VinVin
M.H. Perrott 29
Summary
Feedback systems offer the benefit of accurate gain at low frequencies- Assumes accurate feedback and high open loop DC gain- Gain-bandwidth product of the closed loop system
equals wunity of the open loop amplifier Accuracy of the closed loop DC gain is reduced with
lower open loop DC gain- Want the open loop DC gain to be much higher than the
desired closed loop DC gain for reasonable accuracy Stability of the closed loop system is often evaluated
using the phase margin criterion- Examines the phase at unity gain frequency of the open
loop response, A(w0) = · H(w0), where |A(w0)| = 1 w0 is approximately the same as the closed loop
bandwidth, wbw